The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, an automatic transmission achieving more speed stages has been developed to enhance fuel efficiency and drivability.
Such an automatic transmission achieving more speed stages is desired to increase power performance and driving efficiency for downsizing of an engine. Particularly, a highly efficient multiple-speed transmission having excellent linearity of step ratios closely relates to drivability of a vehicle (e.g., acceleration before and after gear shifts as well as rhythmical engine speed).
However, in the automatic transmission, as the number of speed stages increase, the number of internal components increase, and as a result, mountability, cost, weight, transmission efficiency, and the like are undermined.
Accordingly, development of a planetary gear train which may achieve better efficiency with reduced components may be important in order to increase a fuel efficiency enhancement through the multiple-speeds.
In this background, an eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission enabling more shift-stages has been developed.
However, we have discovered that since a conventional eight-speed automatic transmission has gear ratio span of 6.5-7.5 (gear ratio span is a factor for securing linearity of step ratios), power performance and fuel economy is not great.
In addition, if an eight-speed automatic transmission has gear ratio span larger than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.